I touched on this in a previous answer.
Vader wants to overthrow the Emperor, but believes that Luke isn't ready to kill him. Instead, he tries to provoke Luke into turning towards the Dark Side. In order to provoke him into fighting, he must taunt Luke, ubt must keep Luke focused on him, not the Emperor. Early in the fight, Vader underestimates Luke. Once he realizes that Luke is actually better than expected, that Luke might actually best him, he starts taking the fight more seriously.
Through the fight, he grows angrier and angrier, but still both trying to defeat his son and to turn him to the Dark Side.
From the 25th anniversary edition of RotJ novelization:
Vader was impressed with Luke’s speed. Pleased, even. It was a pity, almost, he couldn’t let the boy kill the Emperor yet. Luke wasn’t ready for that, emotionally. There was still a chance Luke would return to his friends if he destroyed the Emperor now. He needed more extensive tutelage, first— training by both Vader and Palpatine— before he’d be ready to assume his place at Vader’s right hand, ruling the galaxy.
So Vader had to shepherd the boy through periods like this, stop him from doing damage in the wrong places --- or in the right places prematurely.
For the first time, the thought entered Vader's consciousness that his son might best him. He was astounded by the strength --- not to mention the boy's timing, which was honed to a thought's breadth. This was an unexpected circumstance. Unexpected and unwelcome. Vader felt humiliation crawling in on the tail of his first reaction, which was surprise, and his second, which was fear. And then the edge of the humiliation curled up, to reveal bald anger. And now he wanted revenge.
His anger was layered, now --- he did not want to win if the boy was not battling to the fullest. But if winning meant he had to kill a boy who wouldn't fight... then he could do that too. Only he wanted Luke to be aware of those consequences. He wanted Luke to know this was no longer just a game. This was Darkness.
This accusation really made Vader angry. He could tolerate much from the insolent child, but this was insufferable. He must teach this boy a lesson he would never forget, or die learning. “Once again, you underestimate the power of the dark side …”